Method and apparatus for path selection and wavelength assignment in an optical network

ABSTRACT

A method and apparatus for determining a shortest path between a source node and a destination node in an optical network of nodes interconnected with optical transmission links is disclosed. A wavelength graph is used to represent an optical network as a set of electronic nodes and optical channel nodes corresponding to the network nodes with a set of internal links and optical channel links. The electronic node represents the electronic switching fabric that interconnects OEO equipment within a physical node. A single-source shortest path algorithm (e.g., Dijkstra&#39;s algorithm) is applied to the wavelength graph to determine a shortest path. The transformation of the network representation to include the electronic node greatly reduces the number of links in the wavelength graph and significantly increases the computational efficiency.

BACKGROUND

[0001] There exist numerous algorithms for the standard routing problem,in which a network is modeled as a directed graph, and it is assumedthat traffic from one link can always be relayed to its consecutivelink. Among these algorithms, the most efficient one is the well-knownDijkstra algorithm, which has been adopted for use in many of the commonlink-state routing protocols, such as Open Shortest Path First (OSPF),Intermediate System to Intermediate System (IS-IS), and Private Networkto Network Interface (PNNI).

[0002] An important class of routing problem pertains to dynamicallysearching for an optimal lightpath between two edge nodes of an opticalnetwork. The path consists of one or more optical links, each linkhaving a dedicated wavelength for the path. The wavelength, or opticalchannel, assigned to the path may not be the same on every link. Everytime the wavelength assignment changes along the path, specialwavelength conversion equipment, e.g., an optical-electrical-optical(OEO) conversion device, is required.

[0003] In optical network routing, both routing and wavelengthassignment have to be considered simultaneously. In addition, sincethere may not be a full OEO conversion capability at every node, trafficarriving at a node on one wavelength of a link may not be able to berelayed to another wavelength of a consecutive link of the lightpath.Therefore, the standard graph model cannot be used directly.

[0004] In “Lightpath (Wavelength) Routing in Large WDM Networks”,Chlamtac et. al., IEEE Journal on Selected Areas in Communications, Vol.14, No. 5, June 1996, it is proposed to convert the original graph to anew, matrix-like structure with m rows and N columns (N and m being thenumber of network nodes and wavelengths respectively), with each columncorresponding to a node and each row corresponding to a wavelength.Links in the horizontal direction represent connectivity betweenneighboring nodes, using the available wavelengths on the optical fibersthat interconnect the physical nodes. Links in the vertical directionrepresent connectivity between different wavelengths within a singlephysical node that are made available by OEO transmitters and receivers.The resulting graph is called a “wavelength graph.” Chlamtac et al.further derive a routing and wavelength assignment algorithm, referredto as the Shortest Path Algorithm for the Wavelength Graph (SPAWG), withcomputation complexity of O{(N+m)Nm)}. However, the SPAWG algorithm isunderstood to contain an error and therefore is not practicallyavailable. Moreover, the SPAWG algorithm is not Dijkstra-based and thusis difficult to integrate with standard routing protocols such as OSPF.

SUMMARY

[0005] A method used to find the optimal path in an optical networkshould ensure that sufficient OEO transmitters and receivers, ifnecessary, are available along the path. In addition, the method needsto find the lowest cost path, where the cost typically reflects theusage of equipment and facilities along the path. The method should alsobe simple enough to execute in real time, as the paths need to becalculated in real time as users generate connection requests.

[0006] The present approach enables an optical switch to dynamicallyfind an optimal path, with a wavelength or optical channel and OEOequipment (if necessary) dedicated to the path on each link. Using awavelength graph to represent the optical network, the approachrepresents as an electronic node the electronic switching fabric thatinterconnects OEO equipment within a physical node. The transformationof the network representation to include the electronic node greatlyreduces the number of links in the wavelength graph and significantlyincreases the computational efficiency.

[0007] Accordingly, a method of determining a shortest path between asource node and a destination node in an optical network that has pluralnetwork nodes interconnected with optical transmission links includesrepresenting the network as a uni-directional graph G=<V,E> with Vdefining a set of network nodes and E defining a set of uni-directionaloptical transmission links. The graph G is transformed to a wavelengthgraph G′=<V′,E′> with V′ defining a set of electronic nodes and opticalchannel nodes corresponding to the network nodes in set V and with E′defining a set of internal links and optical channel links. The opticalchannel links correspond to the optical transmission links in set E. Asingle-source shortest path algorithm (e.g., Dijkstra's algorithm) isapplied to the graph G′ to determine a shortest path corresponding to anoptimal path on graph G.

[0008] The transforming features assigning an electronic node to eachnetwork node that represents an electronic switching fabricinterconnecting optical-electrical-optical (OEO) transmitters andreceivers of the network node. Optical channel nodes are assigned toeach network node that represent an optical cross-connect for an opticalchannel available at the network node. For each network node, aninternal link is assigned from the electronic node to each opticalchannel node if an associated OEO transmitter is available for thecorresponding optical channel and an internal link is assigned to theelectronic node from each optical channel node if an associated OEOreceiver is available for the corresponding optical channel. For eachoptical transmission link, an optical channel link is assigned between apair of optical channel nodes of corresponding network nodes if thecorresponding optical channel is available on the associated opticaltransmission link.

[0009] Costs are assigned to the internal links in relation to OEOconversion costs. Costs are assigned to the optical channel links inrelation to costs of the corresponding optical transmission links.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0011]FIG. 1 illustrates an optical network of nodes interconnected withoptical transmission links.

[0012]FIG. 2 is a block diagram of a network node in the network of FIG.1.

[0013]FIG. 3 illustrates a pair of connected nodes.

[0014]FIG. 4 illustrates a transformation of one of the nodes in FIG. 3according to the present approach.

[0015]FIG. 5 illustrates a transformation of another of the nodes inFIG. 3 according to the present approach.

[0016]FIG. 6 illustrates interconnection of the transformed nodes ofFIGS. 4 and 5.

[0017]FIG. 7A illustrates a first lightpath that connects through thenodes of FIG. 3.

[0018]FIG. 7B illustrates the lightpath of FIG. 7A transformed accordingto the present approach.

[0019]FIG. 8A illustrates a second lightpath that connects through thenodes of FIG. 3.

[0020]FIG. 8B illustrates the lightpath of FIG. 8A transformed accordingto the present approach.

[0021]FIG. 9 is a flow diagram illustrating a method of the presentapproach.

DETAILED DESCRIPTION

[0022]FIG. 1 illustrates an exemplary optical network. The networkincludes network nodes 10-A, 10-B, 10-C, 10-D interconnected withoptical transmission links 20. A lightpath interconnects edge nodes 12,14 through network nodes A, C and D over links 20 and edge links 22. Thelightpath includes optical channels 24A, 24B, 24C, 24D carried on therespective links. In particular, optical channels 24A and 24B occupywavelength λ₁ and do not require any OEO conversion at node A. At nodeC, OEO equipment 100 converts wavelength λ₁ of optical channel 24B towavelength λ₂ of optical channel 24C. Optical channels 24C and 24Doccupy wavelength λ₂ and do not require any OEO conversion at node D.

[0023]FIG. 2 shows a network node 10 for use in the optical network ofFIG. 1. The network node includes OEO transmitters/receivers 100, anelectronic switching fabric 110, a processor 120 and memory 130. Theswitching fabric 110 provides interconnection of the OEO transmittersand receivers under control of the processor 120. In an embodiment, thememory 130 includes program code for determining an optimal path throughthe network, as described herein.

[0024] The network node 10 further includes an optical cross-connect(OXC) 106 which connect to ports 102-1, 102-2, . . . ,102-N. Each portconnects to an incoming or outgoing transmission link. The OXC 106includes OXC planes 104-1, 104-2, . . . ,104-M. Each of the OXC planesis used to switch or connect data signals from an incoming transmissionlink to an outgoing transmission link using one wavelength. For a paththat uses, e.g., wavelength λ₁ on two consecutive links e and f, trafficis switched from one port (connecting to link e) to another port(connecting to link f) using an OXC-plane λ₁. If a path uses twodifferent wavelengths, e.g., λ₁ and λ₂, on two consecutive links g andh, then traffic arriving from link g passes through OXC-plane λ₁ to theOEO equipment 100, through the switching fabric 110 and back to the OEOequipment, then to OXC-plane λ₂, and leaves the node on link h. Thus, anoptical channel node in a network node represents an OXC-plane for thecorresponding optical channel.

[0025] The present approach uses a wavelength graph to represent anetwork of nodes. In addition, an “electronic node” is defined torepresent the electronic switching fabric that physically interconnectsOEO receivers with OEO transmitters within a physical node. As a result,the number of links in the wavelength graph is greatly reduced, and thecomputational efficiency is significantly increased. A method of thepresent approach is now described.

[0026] An optical network, such as the network shown in FIG. 1, can bemodeled as a uni-directional graph G=<V,E> with V and E representing theset of network nodes and unidirectional links, respectively. The totalnumber of wavelengths available to each network node is given as m. Thepresent approach, as described in the following pseudocode, derives awavelength graph G′=<V′,E′> and finds an optimal path through G′:

[0027] step 1: for any vεV

[0028] define an electronic node v₀εV′;

[0029] for i:=1..m

[0030] define an optical channel node v_(i)εV′;

[0031] if an OEO transmitter is available for optical channel λ_(i)

[0032] define a link (v₀, v_(i))εE′ with a cost of y/2;

[0033] if an OEO receiver is available for optical channel λ_(i)

[0034] define a link (v_(i), v₀)εE′ with a cost of y/2;

[0035] step 2: for any fεE, assume f=(v,w) with a cost of x and v, wεV

[0036] for i:=1..m

[0037] if optical channel λ_(i) is available on f

[0038] define a link (v_(i), w_(i))εE′ with a cost of x;

[0039] step 3: apply Dijkstra's algorithm on G′ to search for a shortestpath, which corresponds to an optimal path on G.

[0040] In the present approach, m optical channel nodes v_(i)(i=1,2, . .. ,m) and an electronic node v₀ are defined for any node v in V. Opticalchannel node v_(i) represents OXC plane λ_(i) of node v. Electronic nodev₀ represents the electronic switching fabric in node v. An internallink is defined from v_(i) to v₀ if and only if there is an availableOEO receiver for optical channel λ_(i). Likewise, an internal link isdefined from v₀ to v_(i) if and only if there is an available OEOtransmitter for optical channel λ_(i).

[0041] If there is an optical transmission link fεE connecting fromnetwork node v to network node w, then for any wavelength available onlink f, say, λ_(i)(i=1,2, . . . m), an optical channel link is definedfrom optical channel node v_(i) to w₁. The cost of each optical channellink is defined as the corresponding physical link cost.

[0042] In this way a new graph G′ is obtained, with costs of bothphysical links and OEO conversion on graph G converted to costs of onlylinks (i.e., internal and optical channel) on graph G′. Therefore, anypath consisting of physical nodes with a wavelength assigned to eachlink is converted to a lightpath consisting of optical channel nodes andpossibly electronic nodes.

[0043] Having transformed the graph G to a wavelength graph G′, thestandard Dijkstra algorithm can be run on graph G′ to search for ashortest path, which corresponds to an optimal path on the originalgraph G. It should be understood that other single-source shortest pathalgorithms, e.g., Bellman-Ford, can be applied to search for a shortestpath on graph G′.

[0044] The foregoing approach to transformation of the wavelength graphis now described with an example. FIG. 3 illustrates an example of twonodes R, SεV with m=4 that are connected by unidirectional physical linkk having link cost x. In this case, only optical channels λ₁ and λ₄ areavailable on the link k.

[0045]FIG. 4 illustrates the transformation in graph G′ of node Rwherein all OEO transmitters and receivers are available. The node R isrepresented as optical channel nodes R₁, R₂, R₃, R₄ and electronic nodeR₀. The full interconnection between the optical channel nodes and theelectronic node within node R is indicated by internal links 20A eachhaving link cost y/2.

[0046]FIG. 5 illustrates the transformation in graph G′ of node Swherein OEO transmitters and receivers are only partially available. Inparticular, only OEO transmitters for optical channels λ₂ and λ₃ areavailable, and only OEO receivers for optical channels λ₁ and λ₃ areavailable in node S. Note that there is no OEO equipment available foroptical channel λ₄. Rather, OXC-plane λ₄ is used to switch traffic fromincoming transmission links to outgoing transmission links withoutwavelength conversion.

[0047] The node S is represented as optical channel nodes S₁, S₂, S₃, S₄and electronic node S₀. Interconnection between the optical channelnodes and the electronic node within node S is indicated by internallinks 20B each having link cost y/2. The internal link cost y/2 can varylink by link within a node and can vary from node to node. However, inmost cases, the same OEO equipments are used in the same node andtherefore, costs associated with them are the same. It is more likely tohave such costs vary from node to node.

[0048] Since there is no OEO equipment available for optical channel λ₄,there are no corresponding internal links between optical channel nodeS₄ and electronic node S₀.

[0049]FIG. 6 illustrates the interconnection of network nodes R and S asrepresented in the transformed wavelength graph G′. In particular,optical channel links 20C are indicated between optical channel nodes R₁and S₁ and between optical channel nodes R₄ and S₄, respectively, eachhaving link cost x. The optical channel link cost x can vary link bylink between the same nodes and for different transmission links.

[0050] The interconnection between optical channel nodes shown in FIG. 6is consistent with the configuration of FIG. 3 wherein only wavelengthsλ₁ and λ₄ are available on optical transmission link k.

[0051]FIG. 7A shows a lightpath that connects from a source node to adestination node through a network that includes the physical nodes Rand S of FIG. 3. In particular, an optical channel received at node R onwavelength λ₃ is converted to wavelength λ₁ for connection to node S. Atnode S, wavelength λ₁ is converted to wavelength λ₂.

[0052]FIG. 7B illustrates the lightpath of FIG. 7A transformed to thewavelength graph G′ according to the present approach with wavelengthconversions indicated at network nodes R and S. In particular, thewavelength conversion from λ₃ to λ₁ is indicated as successive linksfrom optical channel node R₃ to electronic node R₀ and from node R₀ tooptical channel node R₁. Likewise, the wavelength conversion from λ₁ toλ₂ is indicated as successive links from optical channel node S₁ toelectronic node S₀ and from node S₀ to optical channel node S₂.

[0053]FIG. 8A shows a second lightpath that connects from a source nodeto a destination node through a network that includes the physical nodesR and S of FIG. 3. In particular, an optical channel received at node Ron wavelength λ₄ passes to node S without experiencing wavelengthconversion at either node. FIG. 8B illustrates the lightpath of FIG. 8Atransformed to the wavelength graph G′ according to the presentapproach. In particular, since there is no wavelength conversion at thenodes R and S, the links are simply shown as connecting from opticalchannel node R₄ to optical channel node S₄.

[0054]FIG. 9 is a flow diagram illustrating the method of the presentapproach that can be a process performed in one or more network nodes orby a separate network management computer. At 200, information relatingto the topology of the network including links and costs is initializedor retrieved from a data base. At 202, the network topology isrepresented as a graph G=<V,E>. The graph G is transformed to awavelength graph G′=<V′,E′> as defined above at 204. At 206, an optimalpath is determined by applying the Dijkstra algorithm to the wavelengthgraph G′.

[0055] The computation complexity of the present approach isO{(mL)×log₂(mN)}, where m is the number of wavelengths, L is the numberof links, and N is the number of nodes. In this manner, the presentapproach corrects and improves the SPAWG method so that a lightpath canbe found efficiently and optimally. Since the present approach isDijkstra-based, it can be integrated easily with standard link-staterouting protocols such as OSPF.

[0056] The present approach can be used in any optical network with orwithout OEO conversions, including the two extreme cases in which anoptical network either does not have OEO capability at all or has fullOEO capability. On the other hand, an optical network with partial OEOcapability combines the advantages of the two extreme networks, andachieves cost effectiveness and low blocking probability at the sametime. It is for this type of network that the proposed algorithm isparticularly suited.

[0057] The approach for determining the optimal path through an opticalnetwork can be used for both connectionless and connection-orientednetworks. In a connection-oriented network, routing decisions need to bemade at connection setup. The present approach can be used topre-compute paths from any source to any destination. The pre-computedpaths can be stored in memory or a data base for access upon eachconnection request. However, after the next connection request that usesa pre-computed path, the optical channel and OEO availabilityinformation may have changed and thus the pre-computed paths may nolonger be usable. The paths typically need to be recalculated after eachsuccessful connection establishment.

[0058] In a connection-oriented environment, routing decisions are madeat a lower frequency, compared to a connectionless network in which arouting/forwarding decision typically needs to be made for every packet.

[0059] In a preferred embodiment, a path calculation is made wheneverthere is a new connection request, as the frequency of new connectionrequests is typically very low.

[0060] In any case, an update of network topology including changes tolink costs and OEO/optical channel availability information is neededregularly. These update functions can be provided either in acentralized manner using a separate management station that physicallyconnects to every network node, or in a distributed manner by each ofthe network nodes using standard routing protocols such as OSPF orIS-IS.

[0061] It will be apparent to those of ordinary skill in the art thatmethods disclosed herein may be embodied in a computer program productthat includes a computer usable medium. For example, such a computerusable medium can include a readable memory device, such as a hard drivedevice, a CD-ROM, a DVD-ROM, or a computer diskette, having computerreadable program code segments stored thereon. The computer readablemedium can also include a communications or transmission medium, such asa bus or a communications link, either optical, wired, or wireless,having program code segments carried thereon as digital or analog datasignals.

[0062] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A method of determining a shortest path between asource node and a destination node in an optical network having pluralnetwork nodes interconnected with optical transmission links, the methodcomprising: representing the network as a uni-directional graph G=<V,E>with V defining a set of network nodes and E defining a set ofuni-directional optical transmission links; transforming the graph G toa wavelength graph G′=<V′,E′> with V′ defining a set of electronic nodesand optical channel nodes corresponding to the network nodes in set Vand with E′ defining a set of internal links and optical channel links,the optical channel links corresponding to the optical transmissionlinks in set E; and applying a single-source shortest path algorithm tothe graph G′ to determine a shortest path corresponding to an optimalpath on graph G.
 2. The method of claim 1 wherein transformingcomprises: assigning an electronic node to each network node, theelectronic node representing an electronic switching fabricinterconnecting optical-electrical-optical (OEO) transmitters andreceivers of the network node; assigning optical channel nodes to eachnetwork node, each optical channel node representing an opticalcross-connect for an optical channel available at the network node; foreach network node, assigning an internal link from the electronic nodeto each optical channel node if an associated OEO transmitter isavailable for the corresponding optical channel and assigning aninternal link to the electronic node from each optical channel node ifan associated OEO receiver is available for the corresponding opticalchannel; for each optical transmission link, assigning an opticalchannel link between a pair of optical channel nodes of correspondingnetwork nodes if the corresponding optical channel is available on theassociated optical transmission link; and assigning costs to theinternal links and the optical channel links.
 3. The method of claim 2wherein the costs assigned to the internal links are related to OEOconversion costs.
 4. The method of claim 2 wherein the costs assigned tothe optical channel links are related to costs of the correspondingoptical transmission links.
 5. The method of claim 1 wherein applyingthe single-source shortest path algorithm includes applying Dijkstra'salgorithm.
 6. A method of determining an optimal path between a sourcenode and a destination node in an optical network having plural networknodes interconnected with optical transmission links, the methodcomprising: assigning an electronic node to each network node, theelectronic node representing an electronic switching fabricinterconnecting optical-electrical-optical (OEO) transmitters andreceivers of the network node; assigning optical channel nodes to eachnetwork node, each optical channel node representing an opticalcross-connect for an optical channel available at the network node; foreach network node, assigning an internal link from the electronic nodeto each optical channel node if an associated OEO transmitter isavailable for the corresponding optical channel and assigning aninternal link to the electronic node from each optical channel node ifan associated OEO receiver is available for the corresponding opticalchannel; for each optical transmission link, assigning an opticalchannel link between a pair of optical channel nodes of correspondingnetwork nodes if the corresponding optical channel is available on theassociated optical transmission link; assigning costs to the internallinks and the optical channel links; and selecting an optimal path byapplying a single-source shortest path algorithm.
 7. The method of claim6 wherein the costs assigned to the internal links are related to OEOconversion costs.
 8. The method of claim 6 wherein the costs assigned tothe optical channel links are related to costs of the correspondingoptical transmission links.
 9. Apparatus for a node in a network havingplural nodes interconnected with optical transmission links, theapparatus comprising: a processor; a memory connected to the processor;and a computer program, in the memory, for determining an optimal pathbetween a source node and a destination node in the network, which:represents the network as a uni-directional graph G=<V,E> with Vdefining a set of network nodes and E defining a set of uni-directionaloptical transmission links; transforms the graph G to a wavelength graphG′=<V′,E′> with V′ defining a set of electronic nodes and opticalchannel nodes corresponding to the network nodes in set V and with E′defining a set of internal links and optical channel links, the opticalchannel links corresponding to the optical transmission links in set E;and applies a single-source shortest path algorithm to the graph G′ todetermine a shortest path corresponding to an optimal path on graph G.10. The apparatus of claim 9 wherein transforming comprises: assigningan electronic node to each network node, the electronic noderepresenting an electronic switching fabric interconnectingoptical-electrical-optical (OEO) transmitters and receivers of thenetwork node; assigning optical channel nodes to each network node, eachoptical channel node representing an optical cross-connect for anoptical channel available at the network node; for each network node,assigning an internal link from the electronic node to each opticalchannel node if an associated OEO transmitter is available for thecorresponding optical channel and assigning an internal link to theelectronic node from each optical channel node if an associated OEOreceiver is available for the corresponding optical channel; for eachoptical transmission link, assigning an optical channel link between apair of optical channel nodes of corresponding network nodes if thecorresponding optical channel is available on the associated opticaltransmission link; and assigning costs to the internal links and theoptical channel links.
 11. The apparatus of claim 10 wherein the costsassigned to the internal links are related to OEO conversion costs. 12.The apparatus of claim 10 wherein the costs assigned to the opticalchannel links are related to costs of the corresponding opticaltransmission links.
 13. A computer program product for determining anoptimal path between a source node and a destination node in an opticalnetwork having plural network nodes interconnected with opticaltransmission links, the computer program product comprising a computerusable medium having computer readable code thereon, including programcode which: assigns an electronic node to each network node, theelectronic node representing an electronic switching fabricinterconnecting optical-electrical-optical (OEO) transmitters andreceivers of the network node; assigns optical channel nodes to eachnetwork node, each optical channel node representing an opticalcross-connect for an optical channel available at the network node;assigns an internal link from the electronic node to each opticalchannel node if an associated OEO transmitter is available for thecorresponding optical channel and assigning an internal link to theelectronic node from each optical channel node if an associated OEOreceiver is available for the corresponding optical channel; assigns anoptical channel link between a pair of optical channel nodes ofcorresponding network nodes if the corresponding optical channel isavailable on the associated optical transmission link; assigns costs tothe internal links and the optical channel links; and selects an optimalpath by applying a single-source shortest path algorithm.
 14. A computerdata signal comprising a code segment for determining an optimal pathbetween a source node and a destination node in an optical networkhaving plural network nodes interconnected with optical transmissionlinks, the computer data signal including instructions to: assign anelectronic node to each network node, the electronic node representingan electronic switching fabric interconnectingoptical-electrical-optical (OEO) transmitters and receivers of thenetwork node; assign optical channel nodes to each network node, eachoptical channel node representing an optical cross-connect for anoptical channel available at the network node; assign an internal linkfrom the electronic node to each optical channel node if an associatedOEO transmitter is available for the corresponding optical channel andassigning an internal link to the electronic node from each opticalchannel node if an associated OEO receiver is available for thecorresponding optical channel; assign an optical channel link between apair of optical channel nodes of corresponding network nodes if thecorresponding optical channel is available on the associated opticaltransmission link; assign costs to the internal links and the opticalchannel links; and select an optimal path by applying a single-sourceshortest path algorithm.